Charged particle beam systems are used in a variety of applications, including the manufacturing, repair, and inspection of miniature devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Dual beam systems often include a scanning electron microscope (SEM) that can provide a high-resolution image with minimal damage to the target, and a focused ion beam column (FIB) that can be used for imaging or to alter substrates by milling or deposition. In most cases, the SEM and FIB are mechanically coincident, that is, the impact points of both beams are nearly identical.
One common application for a dual beam system is to expose a buried portion of a substrate and then to form an image of the exposed surface. For example, an ion beam can be used to make a vertical cut in a substrate to expose a cross-section of the layers making up an integrated surface, and then an electron beam can be scanned over the newly exposed cross-sectional surface to form an image of it. In a technique referred to as “Slice and View”™, multiple sections are sequentially exposed by the ion beam, with the electron beam forming an image of each section. Slice and View can locate and image small defects in a sample and can be used to produce three-dimensional information about the defect. Another common application of a dual beam is the extraction and thinning of a sample for viewing on a transmission electron microscope.
A high resolution SEM can produce useful images of extremely small features, but aberrations limit the resolution. One source of aberration is “beam interaction,” that is, the spreading of the electrons in the beam due to the repulsive force of the negatively charged electrons. To minimize beam interaction, the distance between the objective lens and the sample, referred to as the working distance, is typically minimized so that the electrons are crowded together for a shorter period of time. One way of reducing the working distance is to bring the magnetic field of the SEM objective closer to the sample. In one type of lens, shown schematically in FIG. 1 and referred to as an axial gap-type lens, the lens 100 produces a magnetic field 102 that extends between two lens poles 104 and 106 above a sample 108. In an axial gap lens, the focusing magnetic field is well above the sample.
In another type of lens, shown schematically in FIG. 2 and referred to as a snorkel-type lens, the lens 200 is cone-shaped and the magnetic field 202 is closer to the sample 204 and so can provide better resolution. In a third type of lens, shown schematically in FIG. 3 and referred to as an immersion lens, the lens 300 produces a significant magnetic field 302, e.g., greater than about 0.025 Tesla, extending to the sample 306, providing still better resolution. The term immersion lens as used herein is not limited to any particular lens structure, but includes any lens that produces a significant magnetic field at the sample. An immersion lens is sometimes referred to as an “ultrahigh resolution lens.” Operating an SEM with an immersion lens is sometimes referred to as operating in “ultrahigh resolution mode,” or “UHR mode.”
It would be desirable to be able to operate the ion beam column and the electron beam column simultaneously, or nearly simultaneously, so that SEM images could be formed rapidly as a sample is processed by the FIB. Unfortunately, the magnetic field produced by the electron lens deflects the ion beam from its intended path due to the Lorentz force on the ions. This force is in a direction perpendicular both to the magnetic field and to the velocity of the ions. Deflections, both inside and outside the ion column, caused by the magnetic field can move the impact point of the ion beam by many hundreds of microns. The deflection is particularly strong when an immersion lens is used, because a significant magnetic field extends all the way to the sample. With an energized lens operating in immersion mode, it has been impossible to operate the ion beam in a dual beam system. FIG. 4 shows the relationship between displacement of a typical FIB beam and the current in the typical magnetic lens of an electron beam column over a typical operating range. As can be seen in FIG. 4, the displacement is linear, and can be hundreds of microns.
One solution is to the deflection of the ion beam by the magnetic field is simply to not operate the electron lens in immersion mode when using the FIB, but non-immersion modes typically have lower resolution. Another solution is to switch the SEM magnetic lens off when using the ion beam and turn it back on when the electron beam is required. Turning the SEM lens on and off, however, creates additional problems. When the lens is turned off, a residual magnetic field remains and decays over time. This time dependent field changes the trajectory of the ions, making it difficult to use the FIB. The magnetic objective lens of an SEM uses significant electrical current and therefore generates a significant amount of heat, the heat being proportional to the square of the current. The heat by an SEM causes components of the dual beam system to expand. The resolution of an SEM, being on the order of magnitude of nanometers, requires a very stable physical platform and the system therefore requires a significant amount of time after being turned on to reach thermal equilibrium and become stable. As the resolution of systems has increased, stability has become more important, and longer waits are required. For example, the Slice and View technique described above requires frequent alternating between the ion beam and the electron beam. Waiting for the system to stabilize after forming each image can significantly increase processing time.
FIB columns in dual beam systems without an immersion lens have been used while the electron lens is on, but the resolution of such systems is less than that of a system with an immersion lens. U.S. Pat. No. 7,411,192 to Takeuchi et al. (“Takeuchi”) describes a compensation system that employs magnetic coils in an ion column to compensate for the magnetic field of a snorkel lens. A snorkel lens, however, produces a weaker magnetic field at the sample than does an immersion lens and has lower resolution. Most ion beam systems do not include magnetic deflections lenses, and so the solution of Takeuchi requires extensively redesigning the FIB column to incorporate the magnetic lenses, and the redesign presents additional problems. Takeuchi also teaches that a compensating magnetic field can greatly reduce the separation of gallium isotopes in the beam.
Although dual beam systems with a magnetic immersion mode SEM have been available for many years, they have been unable to operate the FIB when the SEM is operating in immersion mode.